Queen ants put stop to sperm wars

The seminal fluid of insects like this male leafcutter ant can harm the sperm of their competitors.

Tim Evison

The female of a species of ant that mates with many males, can stop the battle between competing sperm in her reproductive tract, researchers have found.

Biologist Associate Professor Boris Baer from the University of Western Australia in Perth and colleagues report their study of copulation in social insects today in the journal Science.

"We basically show that there are two wars going on at the same time," says Dr Baer.

"The male would actually like to kill sperm from other males, but the female has other ideas."

Dr Baer says research has long focussed on how the properties of sperm, such as how fast the cells swim, affect the reproductive success of males.

But, in recent years, he says, another picture has been emerging.

"What we show here is that the seminal fluid has to be taken into account," says Dr Baer.

He says seminal fluid was traditionally seen as little more than "a spoonful of sugar" that supported the survival of the sperm on its journey to the egg.

But mounting research has shown that seminal fluid has numerous proteins that are involved in all manner of intriguing reproductive battles.

"It's much more complex than we thought originally," says Dr Baer.

Previous research by Baer and colleagues has found seminal fluid of male honey bees contains 67 proteins.

They have documented for the first time in bees and ants that seminal fluid can be used as a weapon to kill off competing sperm from other males that also mate with the same female.

Their experiments showed that the survival of sperm from one male is reduced in the presence of the seminal fluid from a competing male.

Not only that, but female insects can also weigh into this sperm warfare.

Female intervention

In social insects the queen has only one mating season early in her life and must store sperm for use long after the males have died.

In the Atta species of panamanian leafcutter ants, the queen stores more than half a billion sperm from numerous males in her sperm storage organ, the spermatheca, and uses them over several decades to sire 100 million offspring.

It is in her interest to keep this sperm alive, says Dr Baer, perhaps so she can choose sperm from the "best" male or so she can produce offspring with genetic diversity.

Dr Baer and colleagues found that fluid from the queen's spermatheca prevents the ejaculate of one male from harming the sperm of a competitor.

So like a wealthy female spectator in a gladiatorial arena, the queen ant gives the "thumbs up" to keep sperm alive, when it would otherwise be killed off by a competitor's seminal fluid, says Dr Baer.

As part of their experiment, Dr Baer and colleagues developed an artificial insemination system in which they "washed" the queen's spermatheca with salty water and collected her fluids.

They then tested the impact of the spermathecal fluid on sperm to find it cancelled out the effect of competitor seminal fluid on sperm survival.

"It stops the war between the males," says Dr Baer.

In species, like bumblebees where females only mate with a single male, the male inserts a "plug" into the female once she has mated, which explains why a sperm warfare system has not evolved in that species, say the researchers.

Dr Baer and colleagues now plan to study honey bees to see if their queen also intervenes in sperm wars.